In the context of the various advantages and drawbacks of existing energy sources and the ongoing quest to find a “better” (i.e., safer, less-polling, more-efficient, less-expensive, etc.) energy source, interest is growing in a developing nuclear technology involving gas-cooled, “pebble-bed” nuclear reactors. Recent streamlining of the regulatory structure for obtaining nuclear-reactor construction and operating licenses along with new government plans for funding the research and development of promising nuclear-reactor designs further fuel this interest.
Pebble-bed nuclear reactors offer substantial promise and many advantages over conventional liquid-cooled nuclear reactors. Like current commercial liquid-cooled reactors, which produce about 20 percent of U.S. electricity, a pebble bed reactor uses uranium as its power source. Under the right conditions, uranium atoms split, or “fission,” throwing off energetic neutrons and other particles that break up still more uranium atoms in a chain reaction that generates enormous amounts of heat. In liquid-cooled reactors, the heat boils water to create steam to drive turbines and create electricity. In a gas-cooled pebble bed reactor, the nuclear reactions heat helium gas, which spins turbines as it expands.
Compared to standard liquid-cooled nuclear reactors, gas-cooled pebble-bed reactors promise to be safer, cleaner, more-quickly built, cheaper to build and operate, smaller and more efficient. Gas-cooled pebble-bed reactors can be built and safely operated without needing a containment dome, and they can be operated at higher, more-efficient temperatures than liquid-cooled reactors.
A small 15 Megawatt-electric, high-temperature, helium-cooled pebble-bed reactor with a core consisting of uranium-laced graphite fuel pebbles has been successfully operating for more than 20 years in Germany. The basic principles of this reactor are being revisited today with several new designs, which are more powerful, bigger, and yet modular (i.e., can be easily assembled and taken apart). These new reactors may have a major impact on power generation in the not-so-distant future, in the United States and around the world.
The first of the new breed will be the 125 Megawatt Pebble-Bed Modular Reactor (PBMR), currently being built in South Africa by the PBMR consortium led by ESKOM. At the same time, Massachusetts Institute of Technology and the United States Department of Energy's Idaho National Environmental Engineering Laboratory are also working to develop similar technology in the United States, as part of the Modular Pebble-Bed Reactor (MPBR) research project.
Pebble-bed reactors are continuously refueled by slowly cycling radioactive fuel pebbles, which resemble billiard balls, through the reactor core. Altogether, there about 360,000 pebbles in the core. One pebble is discharged from the bottom of the reactor about every 30 seconds, and an average pebble is cycled through the core 15 times before being discarded. The fuel pebbles are believed to be immune to the “worst-case” nuclear-reactor scenario (i.e., a loss of coolant in the reactor core that would lead to a melting of the uranium fuel and a catastrophic release of radiation) because the graphite in the pebbles which encase the uranium cannot get hot enough to melt. This graphite encasement may also make the spent fuel pebbles more rugged and more resistant to corrosion in long-term storage.
In the German reactor, all pebbles in the core are identical. A novel feature of the PBMR (and the MPBR), however, is the use of two different kinds of pebbles: the usual fuel pebble (i.e., a 60 mm graphite sphere containing, e.g., about 11,000 micro-spheres of coated uranium dioxide) and a pure graphite “reflector pebble” of nearly identical size, weight, and surface roughness. The graphite reflector pebbles, like the graphite lining of the reactor, reflect and slow the uranium's neutrons to moderate the energy-producing fission process.
Each of the uranium-dioxide micro-spheres in the fuel particles is typically about 0.5 mm in diameter and is coated with a layer of porous carbon, a layer of high-density pyrolitic carbon, a layer of silicon carbide, and then another layer of high-density pyrolitic carbon. The silicon carbide layer is sufficiently dense that no radiologically significant quantities of gaseous or metallic fission products are released from the fuel elements at temperatures up to 1,650° C.; this temperature range exceeds far beyond the normal operating temperature (about 1,200° C.) of a reactor and is further believed to exceed the core temperature response that would arise from a loss of forced cooling in the reactor.
To provide a self-sustaining reaction, the uranium of the uranium-oxide is enriched to provide about 8% U-235, which is the isotope of uranium that undergoes the fission reaction. The encapsulation of the uranium-oxide micro-spheres also reduces or eliminates any risk that they might otherwise pose as a resource for weapons proliferation, which is low to begin with due to the relatively low concentration of U-235.
As shown in the left panel of FIG. 1, the reflector pebbles 10 are fed through a drop-hole at the end of a central conduit 12 into a reactor core vessel 14. The central conduit 12 leads to a drop hole at the approximate center of the ceiling 15 of the reactor vessel 14, though the central conduit 12 (or set of such conduits) need not enter the ceiling precisely at the center. The reactor core vessel 14 is a cylinder encased in walls of reflecting graphite blocks. Within the reactor core vessel 14, under normal operating conditions, the reflector pebbles 10 fill a central reflector column 16 on the axis of the reactor core vessel 14. Meanwhile, fuel pebbles 18 are fed through drop-holes at the ends of a plurality of peripheral conduits 20 passing through the ceiling 15 of the reactor core vessel 14. Within the reactor core vessel 14, under normal operating conditions, the fuel pebbles 18 form an annular fuel column 22 between the column 16 of reflector pebbles 10 and the outer vessel wall.
The pebbles 10, 18 slowly flow downward through the reactor core vessel 14 to the sorter 26 at its exit. The sorter 26 sorts the reflector pebbles 10 from the fuel pebbles 18 as they exit the vessel 14 and typically redirects the pebbles 10, 18 back to the top of the vessel 14 through conduits 12 and 20 by applying a pressure differential. The sorter 26 further identifies spent fuel pebbles 28, which it removes from circulation, and introduces fresh fuel pebbles 30 to replace the spent fuel pebbles 28.
Within the reactor core vessel 14, the central reflector column 16 and the annular fuel column 22 do not share a distinct boundary as mixing of the reflector pebbles 10 and fuel pebbles 18 occurs in what is referred to as an annular mixed column 24 between the central reflector column 16 and annular outer column 22. The mixed column 24 is not directly controlled. Instead, it arises spontaneously through complicated dynamical processes occurring at the upper surface 34 below the drop holes in the ceiling 15 as well as in the subsurface region 16, 22, 24 of bulk granular flow toward the sorter 26.
The graphite reflector pebbles 10 in the central column 16 help to moderate the nuclear chain reactions in the core vessel 14 by slowing neutrons released from the fuel pebbles 18 and reflecting them back into the fuel column 22 where they can cause more fission events and thus sustain the reaction. In this process, the graphite does not itself undergo any nuclear fission reactions; it simply redirects the neutrons and absorbs some of their kinetic energy. The moderating function of the graphite reflector pebbles 10 is also performed by the graphite lining of the fuel pebbles 18 as well as by the graphite blocks which form the outer wall of the core vessel 14.
The special purpose of the central reflector column 16, as a carefully placed moderator, is to flatten the neutron flux profile of the reactor vessel 14. In the PBMR and MPBR, the core vessel 14 is roughly 3.5 m in diameter (larger than the German reactor core), and were it filled only with fuel pebbles 18, the central region would experience much larger fluxes than the outer region. This would lead to non-uniform burning and, given the size of the reactor, could make controlling the reaction more difficult. The central reflector column 16 of reflector pebbles 10 thus allows for greater fuel efficiency and more-uniform burning.
In a conventional liquid-cooled reactor, control over the neutron-flux profile can be achieved by inserting graphite rods into the core. In a pebble-bed reactor, however, it is not practical to introduce such rods because the granular material in the core resists penetration like a hard solid, even while it is slowly draining. In the PBMR and MPBR designs, the central column of graphite pebbles 16 thus circumvents this difficulty by mimicking the effect of a solid graphite rod in a conventional liquid-cooled reactor.
In practice, the fuel pebbles and reflector pebbles mix to some degree within the reactor. However, until recently there was no in-depth scientific understanding of the statistical dynamics of this type of granular mixing, particularly during very slow drainage. Accordingly, no reliable theories existed for answering even some of the simplest questions about this type of drainage. For example, little was known in detail about any of the following phenomena: how the flow rate changes as a function of hole diameter and other properties of the system; the extent of bulk-particle mixing during granular drainage; whether the mixing is diffusive, and if so, its local diffusion coefficient; and the dependence of mixing on particle properties, such as the size, shape, mass, and roughness of the particles.
During operation of the nuclear reactor, the width of the central graphite column must be carefully determined and controlled. If the column is too wide, then the power output of the reactor is overly reduced. On the other hand, if it is too narrow, the neutron flux distribution is overly non-uniform, leading to inefficient fuel burnup. Moreover, the peak neutron flux could exceed fuel temperature limits.
In current PBMR and MPBR designs, the relative sizes of the graphite reflector column 16 and the fuel column 24 are determined by the placement (in advance) of different dropping points for pebbles at the top of the core vessel (where the various conduits 12, 20 enter the ceiling 15 of the reactor vessel 14)). These dropping points cannot be moved, so the steady-state composition of the core cannot be changed once the reactor is built and in operation. Moreover, it is difficult to accurately predict the precise composition of the core, in particular the structure of the mixed column 24, in advance using only approximate model calculations. The existing designs therefore relied upon approximate model calculations, rather than empirical observation, to optimize the composition, and once the reactor is built, the core composition cannot ordinarily be adjusted to improve power efficiency or to control peak temperatures. This also limits the flexibility to use different types of fuel pebbles because the power distribution cannot be reshaped to conform to new fuel characteristics and limitations, once the reactor is built.